1. Field of the Invention
The invention relates to a method for making micro-electromechanical system (MEMS) devices, more particularly to a method involving forming cover caps on a device wafer through film encapsulating techniques for making MEMS devices.
2. Description of the Related Art
Micro-electromechanical system (MEMS) devices, such as electrostatic accelerometers, sensors, actuators, and condenser microphones, normally include a movable active part or mass suspended in an enclosed cavity. The active part is connected to and is suspended on a surrounding wall through cantilever(s) so as to be suspended and movable relative to the surrounding wall and so as to properly perform a desired function. However, the cantilever(s) is relatively fragile, and tends to be damaged during wafer dicing or die packaging for forming individual chips of the MEMS devices if left unprotected. Two main approaches, i.e., the wafer bonding techniques and the film encapsulating techniques, have been developed to protect the cantilevers and the active parts from being damaged during dicing or die packaging for the production of the MEMS devices. In the wafer bonding techniques, a cover wafer formed with holes is bonded to a device wafer formed with the MEMS dies through wafer bonding techniques. The assembly of the device wafer and the cover wafer is subsequently subjected to dicing so as to form individual MEMS devices which are then packaged to form chip-level MEMS packages. However, the manufacturing costs of the MEMS devices using wafer bonding techniques are relatively high.
Referring to FIGS. 1A to 1D, U.S. Pat. No. 6,936,491 discloses a method using the film encapsulating techniques to form the cover caps 90 (see FIG. 1D) on a device wafer having a device-preformed structure including active parts 92, surrounding walls 95, and cantilevers (not shown). Each of the active parts 92 is surrounded by and is connected to a respective one of the surrounding walls 95 through a corresponding one or corresponding ones of the cantilevers so as to be suspended on and be movable relative to the surrounding wall 95 in a cavity surrounded by the surrounding wall 95. However, the method requires formation of a sacrificial layer 91 of a silicon dioxide (see FIG. 1A) on the device wafer such that the sacrificial layer 91 fills the cavities in the device wafer so as to permit formation of a first encapsulation layer 93 (see FIG. 1B) indirectly on a front side of the device wafer through the sacrificial layer 91 (otherwise the first encapsulation layer 93 would fill the cavities to enclose the active parts 92 when the sacrificial layer 91 is not formed), and further requires formation of holes 930 in the first encapsulation layer 93 (see FIG. 1C) for removal of the sacrificial layer 91 through the holes 930, and closing of the holes 930 thereafter by forming a second encapsulation layer 94 on the first encapsulation layer 93 (see FIG. 1D). As such, the method is relatively complicated, and the cantilever(s) may be damaged due to the stress imparted thereto by the sacrificial layer 91 that encloses the cantilever(s) and the active parts 92. In addition, since the first and second encapsulation layers 93, 94 are formed through deposition techniques, they are relatively thin and thus are likely to collapse during dicing or packaging, thereby resulting in damage to the cantilevers and the active parts 92.